Pressure measurements using a gas-friction vacuum meter are known in the art (see, for example, U.S. Pat. No. 4,395,914 and the art cited therein). These processes use a gas-friction vacuum meter having a rotating body suspended contact-free in a magnetic field in a measuring head, in which, with the aid of drive coils, the rotating body is set into rotation, and is driven at a rotation frequency above a preset minimum rotation frequency.
The rotation speed of the body tends to decrease or does actually decrease as a result of the gas friction and the decrease can be compensated by continuous or intermittent current changes in the drive coils, which are energized with alternating current by a three-phase generator connected to those drive coils.
For detection and measurement of the rotation frequency of the rotating body, rotation speed sensors are positioned and arranged in the measuring head, which further transmit their sensor output signals to an electronic analyzer unit. The latter ascertains or determines the pressure of the gas surrounding the rotating body from the change of the rotation frequency as a function of time.
It is known, for example, to use a gas-friction vacuum meter with a freely-rotating body as a measuring tool for determining the pressure of a gas in the low pressure region, as can be seen for example by a study of German patent DE-PS No. 30 19 315. The measuring range of the known instruments is limited to a pressure range below about 0.10 mbar.
This limitation derives from the fact that the gas friction in a low-pressure high-vacuum environment can be taken to be proportional to pressure below this limit, but in higher gas pressure environments the gas friction increases only weakly with increasing gas pressure.
In gas pressure environments above 0.10 mbar the gas friction is determined essentially by the gas viscosity, which, however, is generally pressure independent. The viscosity however is strongly affected by the temperature. As a result a high pressure temperature- and viscosity-dependent component is superimposed on the comparatively very slight pressure dependence of the gas friction in an undesirable manner, which leads to inaccuracies in the pressure measurement.
The installation of a thermostat, which holds the measuring head at a constant temperature, does not solve the problem, because the magnetic interaction of the rotating body and its rotating field will generate heat irregularly. Some temperature variations are therefore unavoidable.
Consequently the heating of the measuring head and the vacuum chamber enclosing the rotating body occurs by magnetic losses and by electrical power dissipated in the drive coils. The gas surrounding the rotating body therefore has temperature fluctuations in heat exchange with the rotating body and the vacuum chamber, which influence the gas friction. Since variation in the gas friction measured at the rotating body can be caused by a variation in temperature or pressure, or both, the thermal fluctuations in the measuring head hinder the use of the gas-friction vacuum meter for pressure measurements above 0.10 mbar.